1. Field of the Invention
The invention relates generally to patient cardiac defibrillation and, more particularly, to an apparatus and method for accurately detecting a susceptible period during which time it is easier for an electrical shock to terminate ventricular fibrillation so that the heart can start again on a regular heart rhythm.
2. Description of the Prior Art
Ventricular fibrillation, which is clinically characterized as a period of irregular heart rhythm, is a life threatening condition. Current practice is to provide the patient with an electrical shock to the heart which is intended to correct the heart beat. This is accomplished with devices which are called "defibrillators" and these devices can be either external or internal to the patient's body. Implantable defibrillators may be prescribed to patients which have a chronic propensity for ventricular fibrillation events. In practice, after ventricular fibrillation is detected, the defibrillator delivers the shock. Recently, it has been shown that there are periods of time when a patient's heart is more susceptible to defibrillation (see, Hsia et al., "Genesis of Sigmoidal Dose-Response Curve Defibrillation by Random Shock: A Theoretical Model Based on Experimental Evidence for a Vulnerable Window during Ventricular Fibrillation", PACE 13:1326-1342 (1990)). Thus, it would be advantageous to provide a defibrillator which delivers defibrillation energy synchronously with the fibrillation waveforms. U.S. Pat. No. 5,500,008 to Fain seeks to expand on the concepts first proposed by Hsia, and describes an implantable defibrillator which uses synchronized defibrillation.
It would be advantageous clinically if the shock strength required for defibrillation could be reduced. Defibrillation research has utilized the concept of a defibrillation threshold (DFT) to characterize the efficacy of defibrillation. The DFT of an individual heart is defined as the smallest electrical strength which is able to terminate ongoing ventricular fibrillation (VF). This concept has been the standard measurement in defibrillation studies. However, the problem of both high-energy failure and low-energy success during defibrillation has been observed, and can occur randomly due to the probabilistic nature of defibrillation. The fluctuation of the number of excitable cells during VF has been thought to contribute to the uncertainty of defibrillation success when shocks are given randomnly. In the Hsia article referenced above, and in related articles, it was found that a high VF waveform voltage was associated with a greater defibrillation success. This led to the hypothesis that a vulnerable period to defibrillation exists at high VF waveform voltages. In line with this hypothesis, U.S. Pat. No. 5,500,008 to Fain, which is herein incorporated by reference, uses a running average of the absolute values of the peaks and troughs on the fibrillation voltages. However, it would be desirable to improve this method.